Directional microphones have openings to both the front and back volumes and provide an output corresponding to the subtraction of two time delayed signals (i.e., the principle of directivity), resulting in a 6 dB/octave low frequency roll-off in their frequency response curves. Compared to pressure or omnidirectional microphones, the output for directional microphones is attenuated by the effective subtraction of the two input signals, while the noise is magnified by the presence of an essentially infinite rear or back volume. Therefore, the signal-to-noise ratio of directional microphones is much poorer at low frequencies, which makes them more sensitive to low frequency noise sources, like wind noise. A brief explanation of the properties of wind provides a better understanding of the problems that wind creates in directional microphones.
Air molecules are always in motion, but usually in a random direction. During a wind, the air molecules have an appreciable bias towards one direction. When an obstacle is met, the air is redirected. Sometimes the velocity of the air is decreased when an obstacle is met. For some obstacles, however, the velocity of the air increases and the air is diverted. The diverted air may produce a vortex where the air swirls in a circular motion. This vortex can have very high wind velocity and pressure. The sound produced by this vortex is usually of low frequency and acts as though it were coming from a point source in the vicinity of the vortex. For a low frequency point source, the phase difference at two loci close to the sound origin will be very small. The amplitude difference, however, can be very large.
Now consider the effect of a vortex caused by the presence of a directional microphone. The output of a directional microphone is related to the displacement of the diaphragm, which reacts to a difference in sound pressure between the front and back volumes. As said above, the turbulence of the wind causes a source of noise that is essentially a point source of low frequency sound at the center of the vortex. The signals received at both sound inlets will then be appreciably in phase, because the frequency is low and, therefore, the wavelength much greater than the spacing between the sound inlets. If the distance between the sound inlets is approximately the same distance as the distance from the closer inlet to the vortex, however, the further inlet will receive a sound 6 dB lower in level than the one arriving at the closer inlet. It is the pressure difference that causes the problem and results in a diaphragm displacement in the direction of the lowest pressure which, consequently, results in a relatively high microphone output. In effect, the directional microphone becomes a close-talking microphone for the wind turbulence, yet remains a directional microphone for plane wave or distant sounds. The problem is accentuated for wind noise since the amplitude of the sound from the wind can be very high, which may deafen the desired sounds, such as those from speech.
The current solution practiced in many directional hearing aids is to use an open celled foam cap or a protective mechanical flat screen or grid that is applied mostly in the faceplate of the hearing aid to smooth the turbulence. Although this solution appears to be helpful in practice, it has a great impact on the design of the faceplate or shell of a hearing aid since it may require more faceplate area, and/or additional parts, and/or additional production steps for assembly. These mechanical solutions do not, however, entirely solve the problem since the wind still produces an annoying sound to the wearer of the hearing aid. Further, the use of an electronic high pass filter may not be effective in situations where high SPL noise sources cause overload in the input stage of the microphone amplifier. Therefore, the low frequency noise signals should be attenuated before they cause distortion products in the high frequency spectrum. As such, there is still a strong desire in the market to reduce the effects of wind noise in directional microphones.